Photoshop COM automation shows additive latency when multiple requests run concurrently

Answer to the Question

Photoshop’s COM automation runs in a single‑threaded apartment (STA). The UI thread processes one COM call at a time per instance, so concurrent requests to the same Photoshop process are serialised and each must wait for the previous one to finish. There is no public API to prioritise or cancel queued requests. The only practical ways to reduce the additive latency are:

• Run each request in its own Photoshop instance – each instance has an independent STA.
• Throttle the number of simultaneous requests so that only a single script talks to a single instance at a time.
• Batch small operations into a single script to minimise the number of COM calls.

Why This Happens (Confirmed Facts)

• Photoshop’s COM server is not thread‑safe; it accepts one command per STA.
• Multiple scripts launched against the same instance will block each other.
• Launching a new instance per script removes the serialization bottleneck.

Practical Steps

1. Measure baseline latency – run two scripts that open Photoshop, perform a simple operation, and close. If the total time ≈ sum of individual times, serialization is in effect.
2. Isolate instances – modify the scripts to start Photoshop with a distinct process ID (e.g., use psd.exe /nologo or the COM CreateObject with a new instance flag) and retry the test. A noticeable drop in total time confirms the benefit of separate instances.
3. Throttle concurrency – implement a lightweight queue (e.g., a task scheduler or a semaphore) that allows only one script to interact with a shared instance at a time. This keeps memory usage lower than spawning many instances.
4. Batch operations – if possible, combine multiple small edits into a single ExtendScript file so that the COM call overhead is amortised.

Missing Diagnostic Detail

To refine the recommendation, please provide the typical number of concurrent automation requests you intend to run and whether they target the same Photoshop instance or separate ones. This will help decide between full isolation, throttling, or batching.

Things to Watch For

• High memory usage when launching many Photoshop instances.
• Potential crashes if a single instance becomes overloaded.
• Race conditions when scripts share temporary files or registry keys.

Verification Checklist

• Start two scripts, measure elapsed time, verify additive latency.
• Start two scripts with separate instances, verify latency reduction.
• Use a profiler or logs to confirm COM calls are queued on the same thread.